Re: The Earth - The Floating Crust

   This subject is very interesting. Let's discuss the concepts
involved try to clarify some of the ideas.

   The crust has been explored extensively by means of seismic  
waves. Its thickness is an average of about 5 km in oceanic regions and
about 35 km underneath the continents. This occurs because the continental
crust is less dense than the oceanic crust. The boundary between
the crust and mantle was discovered in 1909 by a Yugoslavian seismologist
by the name of Mohorovicic and thus this boundary was named after him (Moho for 
short).
   He discovered that below the crust the velocity of P waves(compressional 
waves) increased abruptly from about 6 km/sec to 8 km/sec.  These velocities 
indicate that the crust is less dense than the underlying mantle.
 
   The principle of isostasy is the idea that the crust is less dense and 
floats on a denser mantle, as a iceberg floats in the ocean. Icebergs float 
because the ice is less dense than the surrounding seawater. The flotation 
occurs because the volume of the ice below the sea surface weighs less than the 
volume of water that it displaces. The crust "floats" because the large volume 
of lesser density crust projects into the denser mantles.  These projections 
are called "roots". This is observed mainly in the continental crust, where the 
difference between crust density and mantle density is more significant.
 
   This idea suggests that there is a liquid on which the crust floats. 
Seismological data shows that the mantle just beneath the crust is solid rock. 
But solid rock, which we know to be strong, only for short periods (seconds or
years) and weak over long periods (thousands to millions of years) and flow 
very slowly like a viscous fluid when forces are applied.
  
   Well, from this, we know that below a mountain range, for instance, there is 
a crustal "root" in the mantle in order to compensate the mass of this mountain.
  
   So, if the mass of a mountain range is depleted by erosion, then the weight 
on the crust is lessened, which means less root (crustal is needed for 
flotation, and the mountain range and root disappear.  That is to say, crust 
and mantle tend to reach an equilibrium state, called isostatic equilibrium. 
This is occurring in Scandinavia at the present time, because the load of a 
continental glacier was removed at the end of the last ice age.
  
   We can use the concept of isostasy to explain the crustal deformation caused 
by the ice cap (like a continental glacier).  This glacier is forming exerting 
a weight on the crust. The crust then sags and a root develops to support the 
ice load isostatically.
 
  I hope this fast explanation has helped you.


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Emilson Pereira Leite